The present invention is in the field of rotational molding; more particularly, the invention relates to rotationally molded articles and a method to rotationally mold articles having a layer made of a composition containing a polyamide and an impact modifier.
Rotational molding, also known as rotomolding, is used in the manufacture of hollow objects from thermoplastics. In the basic process of rotational molding, solid or liquid polymers are placed in a mold. The mold is first heated and then cooled while being rotated about two perpendicular axes simultaneously. Many polymers can be used in the rotational molding process. Popular polymers for this use are polyolefins such as polyethylene. It is also known to use polycarbonates, crosslinkable polyethylene, nylon, and other materials. In selecting the rotational molding grade formulation, care must be taken to assure that there will not be thermal degradation during the heating cycle. A general discussion on rotational molding is given in MODERN PLASTICS ENCYCLOPEDIA 1979-1980, Volume 56, No. 10A, beginning at Page 381.
Many approaches have been taken to improve the physical properties of nylon molding compositions. U.S. Pat. No. 3,264,272, U.S. Pat. No. 3,845,163 and U.S. Pat. No. 4,174,358 illustrate the use of ionic copolymers in polyamide molding compositions to improve physical properties. U.S. Pat. Nos. 4,160,790 and 3,472,916 disclose polyamide molding compositions containing copolymers of ethylene and alkyl acrylate esters for improved physical properties.
Polyamide compositions are known which contain up to about 25 percent by weight of ethylene-ethyl acrylate copolymers and the zinc salt or sodium salt of ethylene methacrylic acid copolymer. In these compositions, there is more ethylene-ethyl acrylate copolymer than the salt of ethylene methacrylic acid copolymer. Specifically, these compositions contain from 13 percent to 23 percent by weight of ethylene-ethyl acrylate, and from one percent to 12 percent by weight of the zinc salt or sodium salt of ethylene methacrylic acid copolymer.